At present, the most diverse kinds of electrical devices are actuated by using an electric current.
For example, if an electrical device is supposed to take on a different state in dependence on certain metered values and/or at different times, the actuating of the electrical device can occur by using temporally varying control signals. Then, depending on the type of control signal imposed, the electrical device as a rule takes on a particular state. If is possible for there to be a certain length of time (a so-called time delay) between the imposing of a control value that is necessary or appropriate for a particular state of the electrical device and the eventual adopting of the corresponding state. The size of the time delay may depend not only on the kind and design of the electrical device, but also in particular on the required size of the change in state of the electrical device.
One problem with such an actuating of electrical devices is, in particular, that one would like to know when (and perhaps whether) the electrical device takes on the desired state. Thus, for example, it can happen that the electrical device cannot even adopt the desired state, for example, due to a technical flaw or due to an unusually strong countervailing force. But even when the electrical device eventually adopts the desired state at a particular time, the time delay can be different in length. But for many applications it is desirable to have (or not exceed) a particular, definite delay time.
Thus, in many cases it proves to be not enough to have a simple actuating of an electrical device with a time-varying control signal.
One way [of obtaining] information as to the current state of the electrical device is to provide special metering sensors that report the current state of the electrical device to a control unit. However, in many instances this technique is problematical, since the metering sensors often involve a sometimes sizeable expense outlay. Furthermore, due to tight space availability, the installation of a metering sensor is often impossible or only extremely problematical.
Another fundamental possibility in many cases is to use the time variable control signal itself, which is used for actuating the electrical device, for at least approximate detection of the state of the electrical device (and thus to use the electrical device itself as a kind of “intrinsic metering sensor”). Now, the measuring device (or parts thereof) can be moved from certain especially undesirable (such as especially cramped) areas to areas less critical in regard to the construction space. With such a method, it is sometimes possible to resort to already existing components, or those which have to be provided any way, which can reduce the costs.
One example among many of such electrical devices are electrical actuators for valves, such as are used for motors and pumps, especially for internal combustion engines, hydraulic motors, compressors and hydraulic pumps, or for switching applications in various hydraulic systems or fluid-carrying systems. In such technical devices, one would like not only to actuate different switching states of the valves controlled by means of the actuators, but also in particular obtain a feedback as to whether a switching process has actually taken place and preferably also when the switchover occurred (especially, when it was completed). With a knowledge of such quantities, better control algorithms can be utilized and suitable steps can be taken in event of malfunctions.
Proposals have already been made for electrical actuators whose position can be measured by “intrinsic” sensors.
Thus, for example, a method for estimating the position of a magnetic armature in a coil was proposed by M. F. Rahman, N.C. Cheung and K. W. Lim in the publication “Position Estimation in Solenoid Actuators” in IEEE Transactions on Industry Applications, Vol. 32, No. 3, May/June 1996. The authors describe that the inductance of the coil changes as a function of the position of the magnetic armature arranged movably therein and at first it increases. After a certain time, the inductance drops once more due to saturation. Based on a measurement of the imposed electric current, the authors thus deduce the position of the magnetic armature inside the coil. However, the problem with the method described here is that the proposed measurement method only provides usable results for quasi-stationary systems, as the authors themselves say. But for many technical systems, this limitation does not apply.
Another proposal was made, for example, in the U.S. patent application US 2008/0143346 A1. Here, based on the rising slope of the electric current imposed on an electromagnetic actuator, one infers the position of the actuator. But the method described here requires knowledge of definite starting positions. Starting from these positions, the position change is more or less “integrated up”. Disturbances, such as those in the form of a no longer completely opening or closing valve, can hardly be detected with the proposed method. Furthermore, the method described there is likewise only suited to quasi-stationary systems.
Thus, a need still exists for an improved method for determining a state of an electrical device actuated by a time-varying control signal, especially in regard to the position of an electrically controlled actuator. In similar manner, there is also a need for an improved control device for an actuator.